I'd would like to understand how this kind of fill valve does work:
In this mechanism, the float (ballcock?) pushes on the indicated lever which in turn pushes the diaphragm to block the water flow.
However I believe that contrary to traditional float mechanisms, water pressure then helps pushing on the diaphragm and maintain contact with its housing. This explains the relatively short lever.
The diaphragm itself has a small hole:
What I'd like to understand is the course followed by water when the valve is open and closed, and how pressure is used to help maintaining the valve closed.
More: A view of the diaphragm (seal) in this video.